The present disclosure generally relates to analytical sample processing and, in particular, to a method and an automated system for discriminating bulk liquid from foam which can be present on the bulk liquid of a sample contained in a sample vessel and/or from residuals of the bulk liquid which can be present on the inner side of the sample vessel and/or of a cap for closing the sample vessel.
In automated clinical analyzers, liquid samples, such as bodily fluids, can be tested by various clinical-chemical and immunochemical methods. In practical use, many analytical methods require precise pipetting operations in order to maintain satisfactory analytical accuracy. Usually, pump-controlled probes are used for aspirating and discharging liquids. In order to minimize the danger of cross-contamination and facilitate probe cleaning, it is desirable to position the probe tip just below the liquid surface. The liquid can either be aspirated while keeping the probe stationary or, in case of larger volumes, while lowering the probe further into the vessel so as to maintain the probe tip in the liquid.
In many cases, liquid levels can greatly vary from one liquid vessel to another so that the probe tip has to be reliably positioned within the liquid before starting a pipetting operation. Hence, it is customary to detect the liquid level prior to positioning the probe. In the prior art, liquid level detection is known to be based on various physical principles such as detecting light reflected from the liquid surface or measuring electric characteristics of the probe when put in contact with the liquid.
However, in some cases, especially in the case of liquids which are likely to be subject to foam formation, the reliability of results obtained by conventional liquid level detection techniques can be unacceptably low. For example, when using a technique based on the change of electric capacitance of the probe, the probe is repeatedly charged and discharged using low-frequent electric voltage signals. However, in case foam is present on the bulk liquid, the foam is likely to cause a capacitance change similar to that of the bulk liquid so that there is no clear discrimination between bulk liquid and foam. Accordingly, there is a great risk that the probe will be positioned in the foam instead the bulk liquid resulting in pipetting errors.
Previously, the problem of pipetting errors resulting from foam has been addressed by a method for capacitively determining the uppermost liquid level in which the amount of time required to reach a predetermined voltage value is repeatedly measured for one sample. In order to identify foam, an averaged charging time is requested to exceed a predetermined value. As indicated above, conventional capacitance measurements usually are based on low-frequency voltage signals typically lower than 1 kHz in order to avoid electric impedances. Otherwise, in case of applying high-frequency voltage signals which, for example, are in the range of from 1 MHz to 1 GHz, electric impedances can be generated. Basically, a change of the electric impedance of the probe could be used to discriminate between foam and bulk liquid. However, liquid level detection based on high-frequent impedance measurements requires sophisticated technical equipment and is rather cost-intensive. Furthermore, analyzers based on this technique can cause electric interference effects resulting in low electromagnetic compatibility so that problems with legal provisions may occur. Moreover, in case of employing plural probes, the probes will disturb each other.
Another approach to liquid level detection is given by a measurement of the change of the Ohmic resistance of the probe occurring when the probe tip hits the bulk liquid. While foam can reliably be discriminated from bulk liquid using such technique, the liquid must be in galvanic contact with electric ground which, however, usually is not the case.
Therefore, there is a need to provide an improved method for discriminating between bulk liquid and foam which can reliably be used in case of samples tending to foam formation.